Primary zinc/air batteries are known. They deliver the highest energy density of any commercially available battery system and at a low operating cost. They are usually in the form of small button cells and, as such, are widely used, for example, in hearing aids and in children's toys. They are not rechargeable and at the end of their life they are thrown away.
A typical zinc/air button cell is shown in the self-explanatory FIG. 1A and FIG. 1B of the accompanying drawings which show an axial section of the cell with an enlarged view (FIG. 1B) of part of the cathode. Button cells are normally fabricated with a temporary closure (not shown) externally over the air access passageway or hole, the closure being removed to activate the battery when it is first to be used. The battery functions by admission of air through the air access passageway, the oxygen in the air being consumed by reaction in the cell. The oxygen-depleted air diffuses out of the cell through the air access passageway, and fresh air is admitted.
Whilst it is an attractive feature of zinc/air batteries to be able to supply reactant oxygen simply by provision of an air access passageway in the cathode, this arrangement also has some disadvantages. In particular, it allows transmission of water vapour into and out of the cell and it allows ingress of carbon dioxide. Transmission of water vapour affects the concentration of the potassium hydroxide electrolyte in the anode and the precipitation conditions of the zinc oxide in the anode, cathode and separator, the loss or gain of water over a period depending on the humidity of the environment in which the battery is being used. Carbon dioxide admitted can react with the electrolyte to reduce its activity. In practice, it is believed that the useful life of a zinc/air button cell is determined by gain or (in many cases more usually) loss of water rather than by exhaustion of the electrical capacity.
These problems are addressed to some extent in current zinc/air cells by controlling the water flux by using hydrophobic polytetrafluoroethylene (PTFE) both in the cathode and as a membrane on the cathode, and by careful selection of the size and number of the air access holes. However, neither of these techniques has proved very satisfactory. It has further been suggested to use complex mechanical and/or electromechanical valves to control the admission of air but these are expensive in construction and in operational energy requirement.
Another approach has been to provide in the supporting structure for the battery a tiny electric fan preferably with long air diffusion tubes as set forth in AER WO 94/25991. The fan is actuated when current is drawn from the battery, in order to blow in replacement air. However, about 10% of the electrical energy output of the cell is needed to drive the system and it occupies about 10% of the cell volume. For these reasons and because of cost, it is not an attractive solution to the problem of controlling water flux.
US patent specification no. 4439500 to Gibbard et al. describes zinc/air cells provided with a gas switch to control the water flux into and out of the cell. The switch comprises a liquid disposed in and normally closing an air passageway for the cell, the liquid and passageway being such that when there is an adequate pressure differential across the passageway, the liquid is temporarily forced aside and out of the passageway by the differential air pressure to open the passageway to allow air to flow therethrough into and out of the cell. When the pressure differential subsequently reduces, the liquid returns into the passageway to once more occlude it. Whilst this device is simple, it has a number of disadvantages not least of which is the fact that if the battery is subjected to shock or vibration, for example, the liquid can be permanently displaced from the passageway so that the gas switch is then effectively destroyed. The liquid can also be absorbed by the surrounding structure or contaminate the active materials of the battery.
Another method of controlling air access (and hence water flux) to zinc/air batteries, which is simple and economic, reliable and extremely effective, needs to be developed.
Other air electrode batteries are known, for example air recovery (also known as air assisted or air restored) batteries. An air recovery battery is a battery that uses air to recharge its cathode during periods of low or no discharge. One type of air recovery battery employs zinc powder as the anode, manganese dioxide (MnO2) as the cathode, and an aqueous solution of potassium hydroxide as the electrolyte. At the anode, zinc is oxidized to zincate and at the cathode, MnO2 is reduced to manganese oxyhydrate.
When the cell is not in use or when the rate of discharge is sufficiently slow, atmospheric oxygen enters the cell and reacts with the cathode. Manganese oxyhydrate is oxidized to form MnO2. During high rates of discharge, air recovery batteries operate like conventional alkaline cells by reducing “fresh” (unreduced) MnO2. During low rates of discharge and periods of rest with no current flow, the “consumed” (reduced) MnO2 is restored or recharged by atmospheric oxygen to the fresh state. In air recovery batteries, the cathode is normally housed within a container (for example a can) and at least one air access passageway is provided in the container to allow air to enter and contact the cathode. The provision of an air access passageway in air recovery batteries can give rise to the same or similar problems as arise in zinc/air batteries as described above, and a method of controlling the air access in such batteries, which method is simple and economic, reliable and effective, needs to be developed.